Miniaturization and robustness are key factors in today's electronics industry when considering manufacturing complex devices. Anisotropic conductive adhesives (ACA) and Anisotropic conductive films (ACF) that pass electricity along only one axis provide electrical connection in many critical electronic systems. This approach can replace traditional methods, like soldering, and provides connectivity where conventional technologies fail. ACA/ACF also facilitate a more efficient use of the board ‘real estate’ as well as more flexible and reliable interconnects. Typical ACA pastes contain electrically conductive metallic particles, ranging in size from 2 to 10 micrometers, incorporated in an insulating binder. Larger particles are used in applications such as Ball Grid Arrays (BGA) and Chip Scale Packaging (CSP). Typically, BGA/CSP applications require a particle having a diameter in the range of 100 μm to 1 mm.
When used as conductive fillers, metal coated polymer spheres offer a lower consumption of metal and a larger bonding surface due to their deformability. The deposited metallic shell consists typically of an inner Ni layer and an outer Au layer. The former represents a cost effective electrical conductor while the latter enhances electrical conductivity and corrosion resistance. See for example, US2006/035036A1.
The successful deposition of metals, for example Ni, on polymer substrates requires an effective surface activation, as discussed in US2007/0092698A1 and US2007/0202335A1. Despite numerous attempts to develop viable alternatives, the Sn2+/Pd2+ system remains the most widely used approach to activate polymeric surfaces. For example, see US2007/0063347A1.
After successful activation, the Ni coating is deposited typically through an electroless plating process. This step makes possible the deposition of continuous nickel layers onto polymer spheres of various sizes and surface composition. The uniformity, continuity, and compactness of the metallic shell are essential in preventing catastrophic mechanical failures while the ability to tailor the composition of the polymer core/surface provides materials with a broader range of mechanical properties. The current inventors have developed a process which provides metal coated polymer spheres suitable for electronics and particularly ACA applications. To be used in such applications, particles need to be coated with a conductive material and there is a need for improvements in the surface adhesion of the polymer core particles to any conductive material in order to prevent delamination and to ensure uniform plating and fewer surface defects in the final particles.
The process of the invention provides a method for activating the surface of polymer spheres of different size, composition, and surface functionality, which can be followed by metal plating.
The inventors have realised that treatment of polymer particles with polyamines before activation improves surface adhesion and provides a much better surface for activation and hence coating. The particles treated according to the process of the current invention have an improved surface for activation and hence prevent delamination of the eventual coating from the particle surface.
In EP-A-2154182, an aromatic polyester film is surface modified by contact with an amine terminated polyamine. An amide linkage is formed as the amine breaks ester linkages in the polymer backbone.
The actual polymer particle treated with polyamine is not itself new. WO 93/02112 and WO 00/61647 describe the production of polymer particles suitable for use here. The present inventors have realised that surface treatment of such particles or others provides a surface ideal for activation and subsequent coating upon.
U.S. Pat. No. 6,787,233 describes gold coated styrene polymer particles. The particles are formed by palladination of an activated polymer surface. Activation of the polymer surface can be achieved using ethylene diamine. As we note below, the use of diamines does not provide a surface which is well-suited to activation. It is highly surprising that the use of polyamines provides a much improved surface treated particle.